1. Field of the Invention
This invention relates to a pressure-sensitive adhesive sheet which is to be stuck onto an adherend, and in particular relates to a pressure-sensitive adhesive sheet which forms optical parts, such as light-polarizing sheets for example, which are used by being stuck onto the liquid crystal cell of a liquid crystal display.
2. Description of the Prior Art
Light-polarizing sheets can be used by being stuck onto the liquid crystal cells of liquid crystal display devices (LCD). A pressure-sensitive adhesive layer is formed on one surface of the light-polarizing sheet for sticking to another optical part such as a liquid crystal cell for example (hereinafter, such optical parts are typified by "a liquid crystal cell", and a release sheet is stuck onto this pressure-sensitive adhesive layer. Furthermore, a protective sheet (protective film) constructed with a protective sheet base material and a pressure-sensitive adhesive layer is stuck onto the other surface of light polarizing sheet which is opposite to the pressure-sensitive adhesive layer in order to protect the other surface of the light-polarizing sheet.
In this case, the light-polarizing sheet is used by peeling off the release sheet, sticking the exposed pressure-sensitive adhesive layer to a liquid crystal cell and then peeling off the protective sheet.
A base material of the light-polarizing sheet used on a liquid crystal cell generally has a triple-layer structure with a PVA based polarizer sandwiched between two TAC (triacetylcellulose)-based protective films. However, because of the characteristics of these materials, the base material of the light-polarizing sheet has poor dimensional stability. In particular, the base material is likely to experience a pronounced change in dimensions due to contraction caused in high temperature or high temperature and high humidity environments.
Hence, when such a light-polarizing sheet having the base material is stuck onto a liquid crystal cell with a pressure-sensitive adhesive, defects such as air bubbles generated within the pressure-sensitive adhesive layer, lifting of the pressure-sensitive adhesive layer and peeling off and the like are liable to occur.
In the past, the two-liquid crosslinking-type pressure-sensitive adhesives which have a high pressure-sensitive adhesive strength and a high shear strength have been used as the pressure-sensitive adhesive which forms the pressure-sensitive adhesive layer in order to prevent these problems from arising. By using a pressure-sensitive adhesive of this type, it has been possible to provide improvement in respect of the problems such as the lifting and peeling which are caused by contraction of the base material of the light-polarizing sheet. On the other hand, however, since the contractile stress of the base material of the light-polarizing sheet cannot be absorbed and alleviated by the pressure-sensitive adhesive, the distribution of the residual stress in the base material of the light-polarizing sheet becomes uneven. In particular, stress is likely to be concentrated in the outer peripheral portion of the base material of the light-polarizing sheet. As a result, there is a problem in that so called "white marking" is liable to appear with TN (TFT) liquid crystal cells and color unevenness is liable to appear with STN liquid crystal cells. Here, it should be noted that "white marking" means a phenomenon that a black non-transparent portion of a liquid crystal cell becomes partially transparent due to uneven contraction of a base material of a light-polarizing sheet attached to the liquid crystal cell.